This invention relates to a polishing pad for polishing the surface of a target object requiring a high degree of flatness and smoothness such as a semiconductor wafer and a semiconductor device wafer, as well as to a method of producing such a polishing pad. More particularly, this invention relates to a polishing pad suitable for a polishing process by a polishing technology of judging the time of finishing a polishing process and a method of producing such a polishing pad.
In general, the surface of such a target object requiring a high degree of flatness and smoothness is polished by rotating a lapping plate with a polishing pad pasted on its surface, supplying polishing slurry to the surface of this polishing pad and pressing the surface of the target object thereon. Examples of the slurry to be used for such a purpose include those obtained by dispersing abrading particles for mechanically polishing the surface of the target object inside water or a water-based aqueous solution containing glycols or alcohols and further adding thereto a chemical capable of chemically reacting with the surface of the target object. Such a chemical may be appropriately selected, depending on the material comprising the surface of the target object (or the “target surface”). If the target surface is silicon dioxide, for example, potassium hydroxide, tetramethyl ammonium hydroxide, hydrofluoric acid and fluorides may be used. If the target surface is tungsten, iron nitride and potassium iodate may be used, and if the target surface is copper, glycine, quinaldinic acid, hydrogen peroxide and benzotriazol may be used. Particles of alumina, silica, ceria and diamond with average diameter in the range of 0.001–1 μm are used as the polishing particles.
As for the polishing pad, non-woven cloth pads with an uneven surface (formed by the fiber structure of the surface layer) capable of holding or capturing abrading particles on the surface and pads of a foamed material with an uneven surface formed by air bubble gaps exposed to the surface may be used. During the polishing, the abrading particles between the surface of the polishing pad and the surface of the target object are held or captured on the surface of the polishing pad and move relative to the target object by following the motion of the polishing pad. Since the chemical in the polishing slurry reacts chemically with the surface of the target object, the impurities generated on the surface of the target object are mechanically removed such that the target surface is polished. This is referred to as the chemical mechanical polishing.
As a representative example, a semiconductor device wafer is produced by forming a multi-layer wiring structure on a semiconductor wafer by means of the multi-layer wiring technology. In the above, the multi-layer wiring structure is obtained by using a known thin-film or etching technology to pile up membranes made of materials which are different in hardness such as insulating and metallic membranes and forming very small and complicated multi-layer wiring patterns with different widths and pitching by etching. If there is unevenness on the surface after the film-forming process and after the etching process, steps are formed on the membrane due to this unevenness, and since these steps tend to cause short circuits among the wires, a high level of flatness and smoothness is required on the surface after the film-forming and etching processes. This is why the aforementioned chemical mechanical polishing is practiced.
Besides, each of the membranes in the multi-layer wiring structure of a semiconductor device wafer is required to be polished to a specified thickness and this means that the polishing process must be stopped at the precise moment at which the membrane being polished comes to be of a specified thickness. For this reason, a polishing technology that judges the time to stop the polishing (or the time at which the membrane being polished becomes of a specified thickness) is employed for the polishing of a target object such as a semiconductor wafer and a semiconductor device wafer that requires a high level of flatness and smoothness on its surface.
According to this polishing technology, as disclosed in U.S. Pat. No. 5,893,796 and Japanese Patent Publications Koho 3,326,443 and Tokkai 2003-68686, for example, light is applied onto the target surface of a target object being polished and the reflected light beam is received by an optical sensor and monitored such that the moment at which the membrane being polished comes to have a specified thickness.
As shown in FIGS. 10A and 10B, a polishing process according to this technology is carried out by using a lapping plate 31 on the surface of which is pasted a polishing pad 20 as shown in FIG. 11 through an adhesive 22 applied to its back surface, as in the case of chemical mechanical polishing explained above. As the lapping plate 31 is rotated in the direction shown by arrow R, polishing slurry is supplied to the surface of the polishing pad 20 through a nozzle 35, and the surface of a target object W held by a holder 34 is pressed thereon and rotated in the direction of arrow r.
Since the change in the reflectivity of light on the target surface needs to be monitored, the lapping plate 31 of the polishing device 30a shown in FIG. 10A is provided with a throughhole 36 which penetrates it in the vertical direction and an optical sensor 33 having a light-emitting element and a light-receiving element is disposed below this throughhole 36, as shown, for example, in aforementioned U.S. Pat. No. 5,893,796 and Japanese Patent Publication Koho 3,326,443.
A lid 32 made of a transparent resin material is affixed to the upper end of this throughhole 36 such that its upper surface is on the same plane as the surface of the lapping plate 31. As shown in FIG. 11, the polishing pad 20 is provided with a window 21 at a position corresponding to the throughhole 36 through the lapping plate 31.
With the polishing device 30b shown in FIG. 10B, the optical sensor 33 similarly provided with a light-emitting element and a light-receiving element is disposed inside an indentation with an opening on the surface of the lapping plate 31, as shown, for example, in aforementioned Japanese Patent Publication Tokkai 2003-68686. A similar lid 32 made of a transparent resin material is affixed at the opening of this indentation with its upper surface on the plane as the surface of the lapping plate 31. The polishing pad 20 is provided with a window 21 at a position corresponding to the optical sensor 33 attached to the lapping plate 31 inside the indentation.
Such a prior art polishing pad has an approximately elliptical slot formed so as to completely penetrate it in the vertical direction, as shown in FIG. 11. According to the disclosures in aforementioned U.S. Pat. No. 5,893,796 and Japanese Patent Publications Koho 3,326,443 and Tokkai 2003-68686, the window 21 is formed in such a shape that would fit the slot such that the pad is attached by inserting the window into this slot. In other words, prior art polishing pads as explained above require the cumbersome processes of forming a slot therethrough, producing a window that would correctly match this slot in shape and inserting the window into the slot through the polishing pad.
There are other problems with the prior art. Since a window made of a harder resin material is attached to the elastic polishing pad made of an unwoven cloth material or a foamed material, there is a difference in hardness between the polishing pad and the window. As a result, a difference appears in the force acting on the window and the portions of the polishing pad near the window during a polishing process such that the surface portions of the polishing pad near the window come to be distorted or cracked. Thus, the force securing the window becomes weaker and the window may become displaced or the polishing pad may become destroyed, causing the polishing slurry to leak to the backside of the polishing pad and to adversely affect the force with which the polishing pad sticks to the lapping plate. Since there is the aforementioned difference in hardness between the polishing pad and the window, there is also a difference between them in the degree of surface wears and a step may appear on the surface of the polishing pad. Such a step tends to cause scratches and waviness on the surface and the target surface cannot be polished evenly thereby.
In the case of an elastic polishing pad made of an unwoven cloth or a foamed material, furthermore, since the target object being polished sinks into the polishing pad and undergoes a large local deformation during the polishing process, the surface of the polishing pad cannot be applied stably and uniformly all over the surface of the target object and hence the surface of the target object cannot be polished uniformly.
Thus, a prior art polishing pad cannot stably polish the surface of a target object smoothly and flatly by using the aforementioned polishing technology for judging the time for ending the polishing.